Pipelines for transmitting petroleum and natural gas, etc. are laid after making a jointed pipe by welding steel pipes with their end faces being abutted against each other. In such a case, it often happens that the external and internal surface geometries of an end portion of one steel pipe may not completely match with the external and internal surface geometries of an end portion of the other steel pipe so that deviation, that is, misalignment occurs. Such misalignment is caused by the variations within a tolerable range of quality standard with respect to outer diameter, inner diameter, and wall thickness in a circumferential direction at an end portion of a steel pipe produced under specific Quality Standard. Moreover, the misalignment is also caused by the ovalizing of the external and/or internal surface geometries and the eccentricity of the wall thickness within the quality standard.
A large misalignment at a weld zone which is butt welded may cause a failure originated from a weld zone in a pipeline. To minimize the risk of failure, it is necessary to limit the misalignment to a minimum. However, the misalignment occurring when two steel pipes are welded with their end portions faces being butted against each other is typically checked only by visual inspection.
Regarding the misalignment occurring when performing butt welding, various proposals have been made in the past, which includes Patent Literature 1, for example. Patent Literature 1 has its objective to prevent the occurrence of buckling in a jointed pipe attributable to misalignment etc. when bending and winding up the butt welded jointed pipe on a drum in the laying of a pipeline by a reel purge method. Patent Literature 1 proposes the preparation of steel pipes as the first and second steel pipes which are to be butt welded, in which the outer diameter difference, the inner diameter difference, and the yield strength difference at their end portions satisfy a predetermined relational formula. It is reported that this allows the suppression of occurrence of external surface misalignment, internal surface misalignment, and buckling attributable to the strength.